I understand ur problem, call drop happens when u r going away from 1800 band, but call is not handed over to 900 band; there may be two cases:
1. both band have individual BCCH
2. common BCCH for both band.
these HO causes are due to radio reasons, so if it is the case, then check hoMarginLev and hoMarginQual, u may have to adjust these parameters to initiate HO.
u have to check hoLevelUmbrella also for HO reasons.
hope this information helps you..